Hydrophilic copolymers and their use in reprography

ABSTRACT

The invention relates to a hydrophilic copolymer having the general structure: ##STR1## in which R 1  denotes H, CH 3  or C 2  H 5 , 
     R 2  denotes an alkyl or cycloalkyl radical having 1 to 17 carbon atoms, 
     R 3  denotes CH 3 , C 2  H 5  or C 3  H 7 , 
     m is 2, 3 or 4 and 
     n, o, p denote the fractions of the monomer units having acidic side groups, neutral, non-ionizable monomer units and monomer units having basic side groups respectively, in mol %, with n+o+p =100 mol %, with the proviso that n and p are each ≧5 mol %, 
     wherein the copolymer is a linear copolymer having a random structure of monomer units. The copolymer is useful as a hydrophilizing agent for lithographic and offset printing plates.

BACKGROUND OF THE INVENTION

The invention relates to hydrophilic copolymers, built up from polymeric chains, which carry acidic and basic side groups, and to the use of these copolymers for hydrophilizing lithographic printing plates.

To produce offset printing plates, suitable layer supports are provided on one or both sides with a light-sensitive layer (resist layer), by means of which a printing image is generated by photomechanical means. After the printing image has been produced, the layer support carries the printing image areas and, in the image-free areas (non-image areas) forms at the same time the hydrophilic image background for the lithographic printing process.

A layer support for light-sensitive layers, suitable for producing lithographic plates, must therefore meet the following requirements. First, the parts of the light-sensitive layer applied to the layer support, which are relatively readily soluble after exposure, must be detachable easily and without residue from the support in a developing process to produce the non-image areas. The support bared in the non-image areas must be highly hydrophilic, i.e., have a high affinity to water, so that it can rapidly absorb and permanently hold water during the lithographic printing step and thus sufficiently repel the greasy printing ink. The light-sensitive layer must also have adequate adhesion to the layer support; even the printing areas of the layer, which have remained after exposure and development, must still adhere sufficiently strongly to the support in order to achieve a long print run.

Foils of aluminum, steel, copper, brass or zinc and also plastic films or paper can be used as the starting material for such layer supports. These raw materials are converted to layer supports for offset printing plates by a suitable treatment of their surface, such as graining, matte chromium-plating, superficial oxidation and/or application of an interlayer. Aluminum, which is probably the most widely used base material for offset printing plates nowadays, is superficially roughened by known methods such as dry brushing, wet brushing, sandblasting, chemical and/or electrochemical treatment or a combination thereof. To increase the abrasion resistance, the aluminum thus roughened can also be subjected to an anodic oxidation in order to build up a thin oxide layer.

In practice, the support materials, especially aluminum based, anodically oxidized support materials, are frequently subjected to a further treatment step before a light-sensitive layer is applied, for improving the layer adhesion, for increasing the hydrophilic character and/or for enhancing the developability of the light-sensitive layers. These include, for example, the methods known from DE-C-907,147 (=U.S. Pat. No. 2,714,066), DE-B 1,471,707 (=U.S. Pat. No. 3,181,461 and U.S. Pat. No. 3,280,734) or DE-A-2,532,769 (=U.S. Pat. No. 3,902,976), which disclose processes for hydrophilizing printing plate support materials based on aluminum which may have been anodically oxidized. These materials are treated with aqueous sodium silicate solution, without or with the use of electric current.

DE-A-1,134,093 (=U.S. Pat. No. 3,276,868) and DE-C-1,621,478 (=U.S. Pat. No. 4,153,461) have disclosed the use of polyvinylphosphonic acid or copolymers based on vinylphosphonic acid, acrylic acid and vinyl acetate for hydrophilizing printing plate support materials based on aluminum which may have been anodically oxidized. The use of salts of these compounds is also mentioned, but not specified in more detail.

The use of complex fluorides of titanium, zirconium or hafnium according to DE-B-1,300,415 (=U.S. Pat. No. 3,440,050) also leads to additional hydrophilizing of aluminum oxide layers on printing plate support materials.

In addition to these most widely known hydrophilizing methods, the use of the following polymers in this field of application has also been described. For example, in DE-B-1,056,931, the use of water-soluble, linear copolymers based on alkyl vinyl ethers and maleic anhydrides in light-sensitive layers for printing plates is described. In addition, those copolymers in which the maleic anhydride component has been reacted partially or fully with ammonia, an alkali metal hydroxide or an alcohol are also mentioned.

DE-B-1,091,433 has disclosed hydrophilizing of printing plate support materials based on metals by means of film forming organic polymers, such as polymethacrylic acid or sodium carboxymethyl-cellulose or sodium hydroxyethylcellulose for aluminum supports or by means of a copolymer of methyl vinyl ether and maleic anhydride for magnesium supports.

According to DE-B-1,173,917 (=UK 907,719), water-soluble polyfunctional synthetic amino/urea/aldehyde resins or sulfonated synthetic urea/aldehyde resins, which are cured on the metal support for conversion into a water-insoluble state, are used for hydrophilizing printing plate support materials of metals.

To produce a hydrophilic layer on printing plate support materials, a) an aqueous dispersion of a modified urea/formaldehyde resin, of an alkylated methylolmelamine resin or of a melamine/formaldehyde/polyalkylenepolyamine resin and b) an aqueous dispersion of a polyhydroxy compound or polycarboxy compound, such as sodium carboxymethylcellulose are applied, according to DE-B-1,200,847 (=U.S. Pat. No. 3,232,783), successively to the support and the substrate c) thus coated is then treated with an aqueous solution of a salt of Zr, Hf, Ti or Th.

In DE-B-1,257,170 (=U.S. Pat. No. 2,991,204) a copolymer which, in addition to units of acrylic acid, acrylate, acrylamide or methacrylamide, also contains Si-trisubstituted vinylsilane units, is described as a hydrophilizing agent for printing plate support materials.

DE-A-1,471,706 (=U.S. Pat. No. 3,298,852) has disclosed the use of polyacrylic acid as a hydrophilizing agent for printing plate support materials of aluminum, copper or zinc.

According to DE-C-2,107,901 (=U.S. Pat. No. 3,733,200), the hydrophilic layer on a printing plate support material is formed from a water-insoluble hydrophilic homopolymer or copolymer of acrylate or methacrylate, having a water absorption of at least 20% by weight.

In DE-B-2,305,231 (=UK 1,414,575) hydrophilizing of printing plate support materials is described, in which a solution or dispersion of a mixture of an aldehyde and a synthetic polyacrylamide is applied to the support.

DE-A-2,308,196 (=U.S. Pat. No. 3,861,917) has disclosed hydrophilizing of roughened and anodically oxidized aluminum printing plate supports with ethylene/ maleic anhydride copolymers or methyl vinyl ether/maleic anhydride copolymers, with polyacrylic acid, carboxymethylcellulose, sodium poly(vinylbenzene-2,4-disulfonic acid) or polyacrylamide.

In DE-B-2,364,177 (=U.S. Pat. No. 3,860,426), a hydrophilic adhesion layer for aluminum offset printing plates is described, which layer is located between the anodically oxidized surface of the printing plate support and the light-sensitive layer and which, in addition to a cellulose ether, also contains a water-soluble salt of Zn, Ca, Mg, Ba, Sr, Co or Mn. The layer weight of cellulose ether in the hydrophilic adhesion layer is 0.2 to 1.1 mg/dm², and the same layer weight is also quoted for the water-soluble salts. The mixture of cellulose ether and salt is applied to the support in aqueous solution, if appropriate with the addition of an organic solvent and/or of a surfactant.

According to U.S. Pat. No. 3,672,966, anodically oxidized aluminum surfaces are, before or during the sealing with hot water, treated with aqueous solutions of acrylic acid, polyacrylic acid, polymethacrylic acid, polymaleic acid or copolymers of maleic acid with ethylene or vinyl alcohol in order to avoid seal deposits.

According to U.S. Pat. No. 4,049,746, hydrophilizing agents for printing plate support materials contain salt-like reaction products of water-soluble polyacrylic resins having carboxyl groups and polyalkyleneimine/urea/aldehyde resins.

In UK 1,246,696, hydrophilic colloids such as hydroxyethylcellulose, polyacrylamide, polyethylene oxide, polyvinylpyrrolidone, starch or gum arabic, are described as hydrophilizing agents for anodically oxidized aluminum printing plate supports.

EP-B-0,149,490 describes, for hydrophilizing, compounds which additionally contain carboxyl groups or carboxylate groups, sulfo groups or hydroxyl groups, apart from amino groups. These compounds have a molecular weight of at most 1000.

The state of the art has also disclosed the use of metal complexes, which contain low-molecular ligands, for hydrophilizing printing plate support materials. Examples of such complexes are: complex ions of divalent or polyvalent metal cations and ligands, such as ammonia, water, ethylenediamine, nitric oxide, urea or ethylenediaminetetraacetate (DE-A-2,807,396 U.S. Pat. No. 4,208,212); iron cyanide complexes such as K₄ (Fe(CN)₆) or Na₃ (Fe(CN)₆) in the presence of heteropolyacids, such as phosphomolybdic acid or salts thereof, and of phosphates (U.S. Pat. No. 3,769,043 and/or U.S. Pat. No. 4,420,549); and iron cyanide complexes in the presence of phosphates and complex formers such as ethylenediaminetetraacetic acid for electrophotographic printing plates having a zinc oxide surface (U.S. Pat. No. 3,672,885).

In EP-A-0,069,320 (=U.S. Pat. No. 4,427,765), a process is described in which salts of polyvinylphosphonic acids, polyvinylsulfonic acids, polyvinylmethylphosphonic acids and other polyvinyl compounds are used as after treatment agents.

In DE-A-2,615,075 (=UK 1,495,895), a process for treating image-bearing offset printing plates with polyacrylamide or a mixture of polyacrylamide and polyacrylic acid is used.

In SU-A-647,142, a copolymer of acrylamide and vinyl monomers is used for hydrophilizing offset printing plates.

DE-C-1,091,433 describes a process for the after treatment of offset printing plate supports with polymers of methacrylic acid, methyl vinyl ether and maleic anhydride.

Acrylamide for the treatment of printing plate supports is also mentioned in DE-A-2,540,561.

For the same purpose, especially for improving the storage stability of printing plates, DE-A-2,947,708 describes, inter alia, Ni salt solutions of acrylamide and acrylic acid and also acrylamide and vinylpyrrolidone.

All the methods described above, are, however, affected by greater or lesser disadvantages, so that the support materials thus produced frequently no longer meet the current demands of offset printing with respect to developer resistance, hydrophilic properties, free running behavior and steady print runs. Thus, after the treatment with alkali metal silicates, which lead to good developability and hydrophilic character, a certain deterioration in the storage stability of light-sensitive layers applied thereto must be accepted, and the print run of a printing plate thus after treated drops drastically.

Although the complexes of the transition metals in principle promote the hydrophilic character of anodically oxidized aluminum surfaces, they have the disadvantage of being very readily soluble in water, so that they can easily be removed during the development of the layer with aqueous developer systems which recently increasingly contain surfactants and/or chelate formers which have a high affinity to these metals. As a result, the concentration of the transition metal complexes on the surface is reduced to a greater or lesser extent, and this can lead to a weakening of the hydrophilic effect.

In the treatment of supports with water-soluble polymers without scope for anchorage, their high solubility, especially in aqueous-alkaline developers such as are predominantly used for developing positive-working light-sensitive layers, also leads to a marked weakening of the hydrophilizing effect.

Monomeric hydrophilic compounds such as those described, for example, in EP-B-0149,490, quite generally have the disadvantage of being washed away relatively rapidly from the bared non-image area surface during the development and printing process and losing their hydrophilic effect, since the anchorage points in the surface are insufficient.

The combination of a mixture of a water-soluble polymer, such as a cellulose ether, and a water soluble metal salt leads, since the layer weights and hence the layer thickness are selected at a relatively high level (see DE-B-2,364,177), to reduced adhesion of the resist layer, and this can manifest itself, for example, in parts of the developer fluid undermining image areas during the development.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a high-polymeric hydrophilizing agent for support materials for offset printing plates which does not adversely affect the storage stability of the light-sensitive layers of the offset printing plates.

Another object of the present invention is to provide an agent which, in addition to good adhesion to the surface of the aluminum support on the one hand and to the light-sensitive layer on the other hand, effects a durable hydrophilic character of the non-image areas of the finished developed offset printing plates.

A further object of the present invention is to provide an agent which is difficult to wash out of the support material treated with it.

Yet another object of the present invention is to provide a process for producing the agent.

In accomplishing the foregoing objectives, there has been provided, in accordance with one aspect of the present invention, a hydrophilic copolymer having the general structure: ##STR2## in which R¹ denotes H, CH₃ or C₂ H₅,

R² denotes an alkyl or cycloalkyl radical having 1 to 17 carbon atoms,

R³ denotes CH₃, C₂ H₅ or C₃ H₇,

m is 2, 3 or 4 and

n, o, p denote the fractions of the monomer units having acidic side groups, neutral, non-ionizable monomer units and monomer units having basic side groups respectively, in mol %, with n+o+p=100 mol %, with the proviso that n and p are each ≧5 mol %,

wherein the copolymer is a linear copolymer having a random structure of the above-described monomers. Preferably, the polymerizable monomers are free-radically polymerizable.

In accordance with another aspect of the present invention there is provided a process for producing the above-described hydrophilic copolymer which comprises the step of free-radically polymerizing the monomers in the presence of a free-radical initiator.

Lithographic and offset printing plates comprising the inventive copolymer as a hydrophilizing agent are also provided.

Other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It is to be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For preparing the polymeric hydrophilizing agents according to the invention, specially selected monomers are copolymerized by means of free radicals in such a way that polymer chains are built up on which acidic and basic side groups, which are ionizable and are capable of salt formation, are arranged in a random distribution.

In between, there are neutral monomer units which do not have ionizable side groups. The acidic groups of the polymeric hydrophilizing agents according to the invention have good adhesion to the aluminum support, while the basic side groups effect an increase in the hydrophilic character by interaction with the fountain solution.

The structure of the polymeric hydrophilizing agents is as follows: ##STR3## in which R¹ : H, CH₃ or C₂ H₅,

R² : alkyl or cycloalkyl radical having 1 to 17 carbon atoms,

R³ : CH₃, C₂ H₅ or C₃ H₇,

m: 2, 3 or 4 and

n, o and p: fractions of the monomer units in mol %, with n+o+p=100 mol %, with the proviso that n and p each are ≧5 mol %.

The values of n and p are in the range from about 5 to 50 mol %, especially between 10 and 40 mol %. In particular, the acidic and basic monomer units are in an approximately equimolar mutual ratio (n≈p) and are each present in the copolymers in a content of about 20 to 50 mol %.

The neutral, non-ionizable monomer units have a fraction (o) from about 0 to 90 mol %; at an approximately equimolar ratio of the acidic and basic monomer units, they are present in a fraction of preferably about 0 to 80 mol %, especially from about 20 to 60 mol %.

The hydrophilic copolymers have a random structure and an average molecular weight of at least 1,000, preferably from about 20,000 to 50,000, but copolymers having molecular weights even higher than 50,000 can also be used with advantage.

Acidic monomer units (monomer type A) are formed from methacrylic acid as the monomer.

Neutral, non-ionizable monomer units (monomer type B) originate, for example, from methyl acrylate, methyl methacrylate, ethyl acrylate and ethyl methacrylate.

Basic monomer units (monomer type C) are formed, for example, by dimethylaminoethyl acrylate and methacrylate, diethylaminoethyl acrylate and methacrylate, dimethylaminopropyl acrylate and methacrylate, diethylaminopropyl acrylate and methacrylate, and dimethylaminobutyl acrylate and methacrylate.

The synthesis of the hydrophilizing agents is advantageously carried out by a polymerization, initiated by free radicals, in organic solvents.

The free-radical initiators used can be the conventional compounds, such as peroxides, for example benzoyl peroxide, or azo compounds, for example azobisisobutyronitrile. In addition, regulators for adjusting the molecular weight can be used. The polymerization in nonpolar solvents, carried out as a precipitation polymerization, has the advantage that low-molecular, oligomeric and chemically inhomogeneous fractions, for example products which contain only a few ionizable groups, remain in solution and can thus easily be separated from the precipitated polymers. This type of polymerization also has the advantage that it is simple and economical. The precipitation polymerization in organic solvents, for example in petroleum spirit fractions in the boiling point range from 100° to 140° C., can be carried out very easily and with good yield in particular of those copolymers in which the ionizable, acid and basic monomer units are each present in approximately equal fractions of about 10 to 50 mol %. If both n and p are in the range from about 1 to 10 mol %, the polymers are isolated by distilling off the organic solvent.

The examples which follow explain the fundamental polymerization method for some preferred copolymers, but the invention is not restricted to the examples given.

The support materials used are preferably aluminum supports such as are described in German Application P 40 23 267.0 filed simultaneously. The manner in which these aluminum supports are coated or surface-treated with the hydrophilizing agents according to the invention is also described in detail in this application. The hydrophilizing agents are anchored in the way of an "absorption", a "complex formation" or a "salt formation" on the specially pretreated aluminum surface in such a way that a permanent hydrophilic character is obtained.

EXAMPLE 1

A copolymer of ethyl acrylate, dimethyl-aminoethyl methacrylate and methacrylic acid is prepared. For this purpose

334 g of ethyl acrylate

524 g of dimethylaminoethyl methacrylate and

287 g of methacrylic acid are dissolved in

1145 g of methyl ethyl ketone (MEK).

1145 g of methyl ethyl ketone, 20% by weight of the solution of monomers described above and 1% by weight of azobisisobutyronitrile (AIBN), relative to the total weight of the monomers, are introduced under a nitrogen blanket gas atmosphere into a three-necked flask provided with a stirrer, reflux condenser, dropping funnel with pressure balance line and a gas inlet tube, and polymerized for 1 hour. Using a dropping funnel, the remaining monomer mixture is added within 1 hour and the whole is polymerized for a further 2 hours under reflux. A further 0.5% by weight of azobisisobutyronitrile is then added and the polymerization is continued for 2 hours under reflux. The product is worked up by filtering off the precipitate formed with suction and washing the precipitate with three times 500 ml of methyl ethyl ketone.

The product is dried at 70° C.

Yield: 83% by weight

The product dissolves in water to give a clear solution.

Completely analogously to the polymer described in Example 1, the copolymers listed in the following table (Examples 2 to 5) are prepared.

    __________________________________________________________________________     Example 2      Example 3                                                                              Example 4                                                                              Example 5                                       __________________________________________________________________________     Monomer                                                                              Methacrylic acid                                                                        Methacrylic                                                                            Methacrylic                                                                            Methacrylic acid                                type A                                                                               13 mol % acid 33 mol %                                                                          acid 50 mol %                                                                          10 mol %                                        Monomer                                                                              Ethyl acrylate 66                                                                       n-Butyl acrylate                                                                               N-Butyl                                         type B                                                                               mol %    34 mol %        methacrylate 80                                                                mol %                                           Monomer                                                                              DMAEMA.sup.1)                                                                           DMAEMA.sup.1)                                                                          DMAEMA.sup.1)                                                                          DMAEMA.sup.1)                                   type C                                                                               21 mol % 33 mol %                                                                               50 mol %                                                                               10 mol %                                        AIBN  1.5% by weight                                                                          2.0% by weight                                                                         1.5% by weight                                                                         1.5% by weight                                        relative to                                                                             relative to                                                                            relative to                                                                            relative to                                           monomers monomers                                                                               monomers                                                                               monomers                                        Solvent                                                                              67 parts by                                                                             67 parts by                                                                            50 parts by                                                                            67 parts by                                     (MEK) weight   weight  weight  weight                                          Yield After stripping off                                                                     90% by weight                                                                          86% by weight                                                                          After stripping                                       the solvent >95%         off the solvent                                       by weight                >95% by weight                                  Solubility                                                                           Soluble in water                                                                        Gives clear                                                                            Gives clear                                                                            Soluble in MEK                                        (pH ≦ 3)                                                                         solution in                                                                            solution in                                                            water   water                                                   __________________________________________________________________________      .sup.1) DMAEMA = Dimethylaminoethyl methacrylate                          

What is claimed is:
 1. A lithographic substrate comprising a support and, applied to at least one surface thereof, a hydrophilizing agent comprising a hydrophilic copolymer having the general structure: ##STR4## in which R¹ denotes H, CH₃ or C₂ H₅,R² denotes an alkyl or cycloalkyl radical having 1 to 17 carbon atoms, R³ denotes CH₃, C₂ H₅ or C₃ H₇, m is 2, 3 or 4 and n, o, p denote the fractions of the monomer units having acidic side groups, neutral, non-ionizable monomer units and monomer units having basic side groups respectively, in mol %, with n+o+p=100 mol %, with the proviso that n and p are each ≧5 mol %,wherein the copolymer is a linear copolymer having a random structure of monomer units, and wherein said acidic and basic monomer units are present in approximately equimolar fractions ranging from about 10 to 40 mol % each, such that (n) is approximately equal to (p), and said neutral, non-ionizable monomer units are present in a fraction (o) ranging from about 80 to 20 mol %.
 2. A lithographic substrate as claimed in claim 1, wherein said alkyl radical R² is methyl, ethyl, propyl, or isopropyl.
 3. A lithographic substrate as claimed in claim 1, wherein said monomer units having acidic side groups are present in a fraction (n) ranging from about 5 to 50 mol %.
 4. A lithographic substrate as claimed in claim 1, wherein said monomer units having basic side groups are present in a fraction (p) ranging from about 5 to 50 mol %.
 5. A lithographic substrate as claimed in claim 1, wherein said neutral, non-ionizable monomer units are present in a fraction (o) ranging from about 0 to 90 mol %.
 6. A lithographic substrate as claimed in claim 1, wherein said acidic monomer units are in an approximately equimolar ratio to said basic monomer units.
 7. A lithographic substrate as claimed in claim 1, wherein said neutral, non-ionizable monomer units are selected from the group consisting of methyl acrylate, methyl methacrylate, ethyl acrylate or ethyl methacrylate.
 8. A lithographic substrate as claimed in claim 1, wherein said copolymer has an average molecular weight of at least
 1000. 9. A lithographic substrate as claimed in claim 8, wherein said copolymer has an average molecular weight from about 20,000 to 50,000.
 10. A lithographic substrate comprising a support and, applied to at least one surface thereof, a hydrophilizing agent comprising a hydrophilic copolymer having the general structure: ##STR5## in which R¹ denotes H, CH₃ or C₂ H₅,R² denotes an alkyl or cycloalkyl radical having 1 to 17 carbon atoms, R³ denotes CH₃, C₂ H₅ or C₃ H₇, m is 2, 3 or 4 and n, o, p denote the fractions of the monomer units having acidic side groups, neutral, non-ionizable monomer units and monomer units having basic side groups respectively, in mol %, with n+o+p=100 mol %, with the proviso that n and p are each ≧5 mol %,wherein the copolymer is a linear copolymer having a random structure of monomer units, and wherein said acidic and basic monomer units are present in approximately equimolar factions ranging from about 10 to 40 mol % each, such that (n) is approximately equal to (p), and said neutral, non-ionizable monomer units are present in a fraction (o) ranging from about 80 to 20 mol %, which is produced by a process comprising the step of free-radically polymerizing said monomers units in the presence of a free-radical initiator.
 11. A lithographic substrate as claimed in claim 10, wherein said polymerization is a precipitation polymerization carried out in a solvent in which said monomers are homogeneously soluble at a temperature from about 60° to 120° C. and in which said copolymer so formed is insoluble and precipitates.
 12. A lithographic substrate as claimed in claim 11, wherein said solvent is a organic solvent.
 13. An offset printing plate comprising an anodized aluminum support and, applied to at least one surface thereof, a hydrophilizing agent comprising a hydrophilic copolymer having the general structure: ##STR6## in which R¹ denotes H, CH₃ or C₂ H₅,R² denotes an alkyl or cycloalkyl radical having 1 to 17 carbon atoms, R³ denotes CH₃, C₂ H₅ or C₃ H₇, m is 2, 3 or 4 and n, o, p denote the fractions of the monomer units having acidic side groups, neutral, non-ionizable monomer units and monomer units having basic side groups respectively, in mol %, with n+o+p=100 mol %, with the proviso that n and p are each ≧5 mol %,wherein the copolymer is a linear copolymer having a random structure of monomer units, and wherein said acidic and basic monomer units are present in approximately equimolar fractions ranging from about 10 to 40 mol % each, such that (n) is approximately equal to (p), and said neutral, non-ionizable monomer units are present in a fraction (o) ranging from about 80 to 20 mol %. 